Today`s post is going to be a little bit unusual: I`m going to ask questions instead of only telling the stories.
So here is the situation: a batch of parts has come to our production facility. According to the claimed manufacturer's process the parts were normalized and tempered.
The exact tempering temperature is unknown, but taking into account the specifics of the parts production and their usage, i suppose that their tempering temperature was somewhere
between 180°C and 400°C. All the parts were black, except the one «outsider»: it was reddish-brown or something like that ... and it defenitely was not the rust.
Fig. 1. Fragments of the parts: left picture – color of the majority of the parts, right picture - color of the «outsider».
We asked the producers of the parts about the reasons of the color change. The answer was: «The regular tempering operation was carried out. The color change is caused
by this thermal operation.»
After such an answer, i have even more questions:
1. Why the rest of the parts subjected to tempering does not have the same reddish-brown color?
2. Was this reddish part subjected to a non-standard tempering? Were there any deviations in tempering temperature or tempering time from a standard regime? Does this «black sheep» have the same properties as the others?
3. Were the other parts actually subjected to tempering?
4. Can the tempering temperature have an effect on the scale color that has already appeared after normalization?
The first three questions can only be answered by the producers. Now I'm interested in the answer to the question number 4: «Can the tempering temperature have an effect
on the scale color that has already appeared after normalization?»
A quick search for any information aobut this phenomena didn`t give me any answers. I didn`d find the answer in metallurgical literature as well. All the papers and
books have only the information about tempering colors which appear on the surfaces free from scale.
Ok, let's think...
Parts necessarily go through normalization. The normalization temperature for the steel in question is about 900-920°C. After normalization scale is formed on the surface. Scale is a mixture of iron oxides: wuistite (FeO), magnetite (Fe3O4) and hematite (Fe2O3). It is known that FeO has a black/gray color, Fe3O4 is black, Fe2O3 in the form of a powder is red. After normalization, the parts have a color close to black, meaning that the scale is mostly composed of wuestite or/and hematite.
Then, after cooling, the parts are transferred to another furnace for tempering.
And depending on the tempering temperature and time, possibly, the transformation of the oxide occurs.
So here are two more questions:
1. What temperature causes the formation of hematite (Fe2O3), so that «repainting» in red occurs?
2. Is this red color really a consequence of the formation of iron oxide Fe2O3?
And, again, I repeat the main question of the post
3. Can the tempering temperature have an effect on the scale color that has already appeared after normalization?